1. Field of the Invention
This invention relates to an oxygen sensor for measuring the oxygen concentration in a gas such as an exhaust gas, and more particularly to an oxygen sensor which can very accurately determine the air-fuel ratio or the like of a gas being measured over a wide range.
2. Description of the Prior Art
To improve the fuel cost and the cleanness of exhaust gas of an automobile engine, it has been proposed to run the engine with an intake air-fuel mixture of lean burn side, i.e., with an air-fuel ratio .lambda., or an excess air ratio, larger than the theoretical optimal value at unity (.lambda.=1). To this end, there is a need for an oxygen sensor which can accurately measure the air-fuel ratio of unity or larger than unity .lambda..gtoreq.1. One example of such oxygen sensors of the prior art was disclosed in Japanese Patent Laying-open Publication No. 130,649/81 which was filed by the Ford Motor Company of the U.S.A.
The above-mentioned Ford's oxygen sensor uses two sintered plates of oxygen-ion-conductive solid electrolyte each of which has electrodes attached to opposite surfaces thereof. One of the sintered plates is used as an oxygen pump element while the other one of them is used as an oxygen concentration cell element. The oxygen pump element and the oxygen concentration cell element are attached to opposite surfaces of a cylindrical spacer so as to sandwich the sidewall of the cylindrical spacer by the two elements. The sidewall of the cylindrical spacer is made of a refractory material and has fine holes bored therein, so that an enclosed space is defined between the above-mentioned two elements while oxygen-diffusing holes are defined by said fine holes of the sidewall of the cylindrical spacer. The oxygen concentration of a gas can be electrically measured by placing the oxygen sensor in the gas, and applying an electric current through the oxygen pump element so as to pump out the oxygen from the above-mentioned enclosed space to the outside atmosphere or the gas being measured, while allowing diffusion of oxygen into the enclosed space through the oxygen-diffusing holes of the cylindrical spacer sidewall, until for instance an oxygen concentration ratio between the enclosed space and the outside atmosphere of the gas being measured reaches a certain stable value. The last mentioned oxygen concentration ratio is given by the oxygen concentration cell element as an output thereof, and the magnitude of the current applied to the oxygen pump element for pumping out oxygen corresponds to the oxygen concentration in the outside atmosphere of the gas being measured. This oxygen sensor uses the oxygen pump element and the oxygen concentration cell element which are separately formed, so that the output from the oxygen sensor has an advantage in that the dependence of the output thereof on the temperature of the outside atmosphere of the gas being measured is low.
However, the above-mentioned sensor of the prior art has shortcoming in that when the accuracy is to be further improved, a heater must be added so that the temperature of the oxygen sensor and the ambient temperature can be kept substantially at a constant level. The addition of such a heater results in a reduced thermal efficiency.